What's your favorite full frame walkaround lens?

Let's say you're going outside to take pictures. You don't know what might come up. What lens do you take? Or, if you cycle between various lenses, which one gives you the best pictures?

My favorite walkaround lens is the 300mm f/2.8 AF-I. It's the smallest and lightest AF-I lens ever made by Nikon, which makes it a joy to carry around. Unlike the 400mm f/2.8, 200-400mm f/4, and 600mm f/4 lenses that everyone else seems to use, it's easy to just cradle the 300mm f/2.8 in your arms and run around while everyone else has to stop, shoulder their monopod + heavier lens combination, and walk.

The fast f/2.8 aperture also means that getting high shutter speeds isn't a challenge even in marginal light, especially with the D600's excellent high ISO performance. Autofocus is extremely good - the AF-I motor snaps things into focus with brutal speed and accuracy. And to top it off, the lens is extremely sharp even wide open. Even with a 2x teleconverter, image quality is still pretty decent.

Of course, the 300/2.8 AF-I's light weight and tiny size comes at a cost - you often don't have enough reach. In order to photograph something, you pretty much have to be so close that you can recognize it with your bare eyes. That's why I always try to carry a 2x teleconverter along too. I often end up regretting it if I don't, because quite often, you can't just walk right up to what you want to photograph (often, things have changed by the time you get there).

Some pictures with the 300/2.8 AF-I:

Oh, and a short footnote: the combo gets even lighter if you mount a D3000 on the 300/2.8 AF-I.